I'll be using an Innovate LM-series wideband 02 setup when my car's back on the road, and this caught my attention given the pump 93 E10 and race fuels I'm likely to be blending w/ it have a range of stoich values.

Well I have seen questions from time to time about adding race gas without tuning which can be a very expensive experiment. I am going to explain how stoich effects AFR. Hopefully in simple format.

I am not going to get into MAF transfer functions or other tuning functions.This is just about fuel.

First lets define a few terms:

AFR: Air Fuel Ratio

Stoich Rating: In simple terms is the AFR to have chemically complete combustion that is neither rich or lean. For Example most pump gasoline is about 14.64 to 1. Which means 14.7 parts of air to 1 part of fuel.

Lambda: Is a term that is utilized when tuning. In basic terms think of it as an adjustment to your stoich rating when tuning AFR. For example with pump gas tuning .80 lambda, 14.7 x .80 = 11.76 AFR target.

So now it is race day. We add some timing to our tune, drain the fuel tank, and fill up with some high octane unleaded VP109.

We go for a blast down the 1/4 mile while datalogging and see the A/F is reading 12.8.

There are a few factors here to consider.

The stoich rating of VP109 is 13.41.

Most wideband A/F modules are calibrated to 14.7 pump gas stoich.>> AS I NOTED ABOVE, 10% ethanol FUEL SUCH AS 93 E10 HAS 14.1 stoich <<

So this is the formula to determine the true A/F for the run.AFR/Wideband Stoich x Race Gas Stoich12.8/14.7*13.41= 11.7 AFR on Race Gas

So you may think 11.7 sounds safe but we must remember the VP109 stoich is 13.41. So 11.7 AFR = .87 lambda. For the most part a safe lambda value is .80 to .82.

So we take VP109 stoich of 13.41 x .80 lambda = 10.73 AFRThis is the AFR (10.73) I am tuning for.

Keep in mind this is not what will display on your wide band as it is programmed to 14.7 stoich. You must convert the AFR.

AFR/Race Gas Stoich*Wide Band Stoich

10.73/13.41*14.7 = 11.76 AFR is what I want to see on my wideband when running VP109.

Just to reiterate 11.7 on the AFR display with VP 109 is really a 10.7 AFR.

Ok so hopefully you are not totally confused.

This is why you have to exercise caution when mixing race gas and pump gas as you do not know the true stoich of the mixture. The leaded race fuels have a stoich that is closest to pump gas. I always drain my tank before putting in the race gas. When I mixed fuels in the past I didn't notice a drastic change in AFR, but when I have 100% unleaded race gas it really leans out with a 14.64 stoich scalar in the tune.

If you know the a/f ratio for stoich for the fuel you are using, you can program your wideband lambda value to reflect that ratio. For that matter, if you consistently use the same fuel, and you find that the engine likes a 13.1 ratio under power, you could set your wideband lambda so that 13.1 : 1 ratio is a perfect “1” lambda. It wont really be “lambda”, but it gives an easy to zero in on target.

Yep, E10's stoich is 14.1:1. It's why most modern vehicles that operate on a range of fuels gets calibrated to lambda instead of AFR. It's easier to just target 0.85 lambda, as an example, and then it is safe regardless of the fuel being used.

Theory is one thing, in the real world racing motors doesn't know or follow theory My message is the weather, track conditions and the age of the fuel being used comes into play, tune for best results and see what YOUR instrument reads and use that tune up for identical conditions later

I'd pick a fuel and run it so you can take it out of the equation. I highly recommend running a mix of pump gas and unleaded racing gas. The leaded race gas works a little better but it will kill your O2 sensors. I'd say pick a fuel, figure out the theoretical correct AFR for that fuel as a starting point, then work from there. You have a lot of new stuff going on with your combo so you'll most likely just need to watch the O2 sensors and then cross check the plugs to see how they look.

Lambda "1" = Stoichiometric, regardless of the fuel being used.At Lambda 1, every fuel particle has found a matching air particle to bond with, meaning there's no excess of either fuel nor air in the mixture.

All of the known fuel AFR-numbers are just calculated from the Lambda-number by 'known' fuel-stoich-numbers.

When mixing fuels, the stoich-number becomes a 'best guess', so it's then better to stick to Lambda-readings because they don't change.

I've mixed leaded/unleaded and my oxy sensors live for years but now I just stick to VP-101 and mix w/91 for cruising and straight when racing. As far as ideal ratios, I agree run it at the track, jet up till it slows and there's your ratio on THAT day in that da which changes and another thing, this cruising in the 15's 16's bs with carbs and BIG duration cams with lots of overlap is NOT the answer like EFI which uses a tps to up pulse widths rates and injects fuel directly on to the intake valves.......... Two different worlds completely just like Dragula is finding out w/my carb and a HEMI and several others also. You want stellar mileage in a old muscle car then put a GEN-3 in it and enjoy the ride........

Brad,The Innovate loggers will show Lamda. This is the easiest, most straightforward to do the testing you want. I know, I know, I haven't switched over myself (yet), but I'm also not looking for the level of difference of you are.

Do your tests looking at lamda, convert it later if you want for curiosity.As BBM stated, Lamda is what the wideband interprets as stoich regardless of the fuel.

I'd avoid pump fuel for your testing. Too many different flavors depending on where and when its bought, never mind batch variations. I'd also either commit to an oxygenated fuel or not. Think about maybe running a Ethanol blend since that's what's in most pump gas (unless you go upstate) and you plan to mix for street driving. yes? Then put a few gallons of non-eth race fuel in the tank for winter storage.